L-amino acids are useful as synthetic intermediates such as pharmaceutical intermediates. Various methods for producing an L-amino acid are known such as an extraction method, a chemical synthesis method, a fermentation method, and an enzymatic synthesis method. The extraction method requires large-scale purification equipment for purification of a target amino acid from protein hydrolysates. Because amino acids produced in the chemical synthesis method are typically racemates, the chemical synthesis method requires an expensive resolving agent, asymmetric catalyst or the like for production of the optically active isomers. The fermentation method produces products at low concentrations, and requires large-scale purification equipment like the extraction method. In addition, the fermentation method is not suitable for synthesis of normatural amino acids. The enzymatic synthesis method can be used to provide a lower-cost, efficient production method that overcomes these problems by using an inexpensive biocatalyst. For example, a method for producing an L-amino acid in which a keto acid corresponding to a target amino acid is reacted with an amino acid dehydrogenase and an enzyme having coenzyme regenerating ability to produce the target L-amino acid (Non-Patent Document 1) is known as an enzymatic synthesis method for producing an L-amino acid. However, the above-mentioned method requires an expensive commercial enzyme or an enzyme purified from a microorganism, and its industrial use has been problematic.
In order to solve such problems, high production methods of these enzymes have been under development in which DNAs encoding these enzymes are cloned using gene recombinant techniques to obtain a transformant, and the transformant is cultured. For example, a method has been known in which a transformant containing a gene encoding a coenzyme-dependent amino acid dehydrogenase and a gene encoding a coenzyme regenerating enzyme is prepared and cultured, and then allowed to act on a keto acid to produce an L-amino acid (Patent Documents 1 and 2). However, the reaction described in Patent Document 1 produces an L-amino acid at a concentration of about 10 mM, which is low for industrial production. In the method described in Patent Document 2, a keto acid is needed to be controlled at a concentration in a reaction fluid of not more than a specific value by adding the keto acid in portions or continuously.
Patent Document 1: JP-A Hei10-23896
Patent Document 2: WO 95/093081
Non-Patent Document 1: Ramesh N. Patel, “Enzymatic synthesis of chiral intermediates for Omapatrilat, an antihypertensive drug”, Biomolecular Engineering, 2001, No. 17, p. 167.